When hydrogen is used as a fuel in motor vehicles, a hydrogen fuel depot infrastructure for refueling must also be developed. Typically, in the use of hydrogen to power fuel cells or internal combustion engines in motor vehicles, hydrogen is stored in on board fuel tanks maintained at a maximum pressure in the range of about 5000 psi. In the instance of a fuel cell powered vehicle, utilization of the hydrogen input into the fuel cell stack occurs at about 30 psi. The consumption and evacuation of hydrogen from on board fuel tanks to power a vehicle having a fuel cell or internal combustion engine should smoothly correlate with a motor vehicle operator's desire for on demand control of the speed, acceleration and other power needs of the operator and vehicle. An increase in the energy efficiency of the overall system of fuel depots and vehicle tanks and their interrelationship is a desirable goal.
Hydrogen powered vehicles may use high pressure hydrogen storage tanks to store hydrogen on board. The use of multiple cylindrically shaped small tanks rather than one large tank is preferred for vehicle design purposes. Current practice is simultaneously to evacuate, as needed, the hydrogen equally from each of the multiple tanks having a parallel outlet relationship. Typically, solenoid outlet valves for all tanks are open simultaneously and the pressure decreases equally in all tanks as the hydrogen is consumed when the vehicle is driven.
Designs for hydrogen refueling stations provide apparatus and systems to replenish hydrogen gas in a motor vehicle tank using a cascade filling process in which there are multiple banks of pressurized hydrogen quantities stored in individual tanks at the station. A first bank (with the lowest pressure) is used to equalize pressure with one or more of the separate tanks in a motor vehicle, then the next bank is utilized to equalize with increased pressure in the vehicle tank, and then the next bank, etc., in a sequence until all banks have equalized in pressure or the fill pressure of the vehicle tank(s) has been reached. In this design, if the first bank at a filling station is at a lower pressure than the vehicle tank(s), then the first bank cannot be used in the filling process; consequently, the next bank at the station with a pressure higher than that of the vehicle tank(s)' pressure must be used. As a result, hydrogen stored in the bank(s) on the refilling station side with pressure lower than the vehicle tank(s) cannot be used to replenish the vehicle fuel supply, and the remaining tanks with higher pressure at the refilling station must be used. This necessary sequence depletes the hydrogen in the higher pressure banks, and does not allow the hydrogen in the lower pressure banks to be fully used.
When a hydrogen powered vehicle is filled with hydrogen, the pressurized storage tanks contain two forms of energy: chemical energy from the hydrogen itself, and mechanical and thermal energy associated with the high pressure under which the hydrogen gas is stored. The mechanical energy from the high pressure under which the hydrogen gas is stored is not utilized when a motor vehicle is driven; thus, the mechanical potential energy from storing hydrogen at high pressure is wasted.